The objectives of the proposed research are (1) to investigate the mechanisms involved in the compensatory proliferative response of the rat intestine after the ileum and/or jejunum are subjected to various stimuli and (2) to provide biological and chemical bases for the responses involved as they relate to the control or regulation of intestinal cell proliferation. The responses of portions of the intestine to a number of treatments including X-irradiation, surgical resection, split courses of resection, and intestinal ischemia plus resection, will be documented and quantified. The magnitude, time of earliest appearance, and the sequence of changes will be studied in the intestinal region most directly affected, but also in adjacent and nearby sites of the gastrointestinal tract. The biological parameters of the intestinal reponse which will be measured include determination of cells/crypt column, crypt to villus ratio, mitotic index, percent of crypts in fission, and radiolabeled thymidine incorporation as measured by dpm/crypt. Other parameters to be measured include intestinal mucosal DNA, hormone levels in plasma and mucosa, and levels of cyclic nucleotides and associated enzymes. Normal intestinal cultured cells will be used as tools to assay for inhibitory factors extracted from villus cells isolated from the mucosa and for other substances thought to influence intestinal growth. The mechanisms involved in crypt fission (the splitting of crypts into two) and the relationship of the process to cell proliferation (or vice=versa) will be studied. The relationship between percent crypt fission (PCF) in 21 day old rats, where PCF is high, and intestinal stem cell number will also be investigated. This will be achieved by appropriate microcolony and macrocolony assays. The crypt fission study will also include an investigation of the distribution of mitotic figures and labeled cells in crypts in different stages of fission. The results should contribute to the understanding of adaptive responses in tissues subject to injury by radiation or surgery. The findings may also be of potential value to patients who undergo massive resection of intestine. Conceivably, there could be an improvement in the understanding of the control of cell proliferation in the intestine with application to tumor control.